pkgs/top-level: check types of nixpkgs.config

This patch simply introduces a plain simple NixOS module and passes
`nixpkgs.config` through it via `evalModules` (with some temporary hackery to
passthru undefined options).

pkgs/top-level/config.nix

@@ -0,0 +1,51 @@
+# This file defines the structure of the `config` nixpkgs option.
+
+{ lib, config, ... }:
+
+with lib;
+
+let
+
+  mkMeta = args: mkOption (builtins.removeAttrs args [ "feature" ] // {
+    type = args.type or (types.uniq types.bool);
+    default = args.default or false;
+    description = args.description or ''
+      Whether to ${args.feature} while evaluating nixpkgs.
+    '' + ''
+      Changing the default will not cause any rebuilds.
+    '';
+  });
+
+  mkMassRebuild = args: mkOption (builtins.removeAttrs args [ "feature" ] // {
+    type = args.type or (types.uniq types.bool);
+    default = args.default or false;
+    description = (args.description or ''
+      Whether to ${args.feature} while building nixpkgs packages.
+    '') + ''
+      Changing the default will cause a mass rebuild.
+    '';
+  });
+
+  options = {
+
+    /* Internal stuff */
+
+    warnings = mkOption {
+      type = types.listOf types.str;
+      default = [];
+      internal = true;
+    };
+
+    /* Config options */
+
+    doCheckByDefault = mkMassRebuild {
+      feature = "run <literal>checkPhase</literal> by default";
+    };
+
+  };
+
+in {
+
+  inherit options;
+
+}